U.S. patent application number 14/231383 was filed with the patent office on 2016-06-16 for temperature control of components on an optical device.
The applicant listed for this patent is Kotura, Inc.. Invention is credited to Dazeng Feng, Jacob Levy, Zhi Li, Wei Qian.
Application Number | 20160170239 14/231383 |
Document ID | / |
Family ID | 54241183 |
Filed Date | 2016-06-16 |
United States Patent
Application |
20160170239 |
Kind Code |
A1 |
Feng; Dazeng ; et
al. |
June 16, 2016 |
TEMPERATURE CONTROL OF COMPONENTS ON AN OPTICAL DEVICE
Abstract
The optical device includes a waveguide positioned on a base and
a modulator positioned on the base. The modulator includes an
electro-absorption medium. The waveguide is configured to guide a
light signal through the modulator such that the light signal is
guided through the electro-absorption medium. A heater is
positioned on the electro-absorption medium such that the
electro-absorption medium is between the base and the heater.
Inventors: |
Feng; Dazeng; (El Monte,
CA) ; Qian; Wei; (Torrance, CA) ; Li; Zhi;
(Alhambra, CA) ; Levy; Jacob; (Sierra Madre,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kotura, Inc. |
Monterey Park |
CA |
US |
|
|
Family ID: |
54241183 |
Appl. No.: |
14/231383 |
Filed: |
March 31, 2014 |
Current U.S.
Class: |
385/2 |
Current CPC
Class: |
G02F 2001/0157 20130101;
G02F 1/015 20130101; H01S 5/12 20130101; G02F 2001/0155 20130101;
G02B 2006/12142 20130101; G02F 1/025 20130101; G02F 2203/60
20130101; G02B 2006/121 20130101; H01S 5/02453 20130101 |
International
Class: |
G02F 1/025 20060101
G02F001/025; H01S 5/024 20060101 H01S005/024; H01S 5/12 20060101
H01S005/12 |
Claims
1. An optical device, comprising: a waveguide positioned on a base
and a modulator positioned on the base, the modulator including an
electro-absorption medium, the waveguide configured to guide a
light signal through the modulator such that the light signal is
guided through the electro-absorption medium; a heater positioned
on the electro-absorption medium such that the electro-absorption
medium is between the base and the heater.
2. The device of claim 1, wherein the modulator is a Franz-Keldysh
modulator that uses the Franz-Keldysh effect to modulate light
signals.
3. The device of claim 1, wherein a ridge of the electro-absorption
medium extends away from the base, the ridge including lateral
sides that connect a top side to a bottom side such that the bottom
side is between the top side and the base, and the heater is
positioned over the top side.
4. The device of claim 3, wherein the heater is positioned on one
or more of the lateral sides of the ridge.
5. The device of claim 1, wherein a ridge of the electro-absorption
medium extends away from the base, the ridge including lateral
sides that connect a top side to a bottom side such that the bottom
side is between the top side and the base, and the heater is
positioned on one or more of the lateral sides of the ridge.
6. The device of claim 1, a ridge of the electro-absorption medium
extends away from the base and is positioned between slab regions
of the electro-absorption medium such that the slab regions of the
electro-absorption medium are continuous with the ridge of the
electro-absorption medium.
7. The device of claim 1, wherein a ridge of the electro-absorption
medium extends away from the base, the ridge including lateral
sides that connect a top side to a bottom side such that the bottom
side is between the top side and the base, and at least a portion
of the heater is less than 2 .mu.m from the ridge.
8. The device of claim 7, wherein at least a portion of the heater
is less than 2 .mu.m from the top of the ridge.
9. The device of claim 1, wherein a ridge of the electro-absorption
medium extends away from the base and no portion of the heater is
more than 2 .mu.m from the ridge.
10. The device of claim 1, further comprising: a light source
positioned on the base such that the light signal includes at least
a portion of the light generated by the light source.
11. The device of claim 10, wherein the light source is a
distributed feedback laser.
12. The device of claim 10, wherein the light source has a channel
wavelength shift rate greater than 0.05 nm/.degree. C.
13. The device of claim 10, wherein the light source generates a
light signal having the most intense wavelength at a design
wavelength when the light source is at a design temperature, the
modulator is to modulate light signal at a modulation wavelength,
the modulation wavelength being equal to the design wavelength when
the modulator is at the design temperature, and further comprising:
further comprising: electronics configured to operate the heater
such that a temperature of the heater is maintained above a
threshold temperature, the threshold temperature being less than or
equal to the design temperature.
14. The device of claim 13, wherein the electronics are configured
to operate the heater such that the heater does not generate heat
in response to a temperature of the modulator being above the
threshold temperature.
15. The device of claim 13, wherein the threshold temperature is
less than the design temperature.
16. The device of claim 13, wherein the modulator efficiently
modulates light having wavelength in an operating bandwidth (OBW)
and the modulation wavelength is the wavelength at a center of the
operating wavelength range.
17. The device of claim 1, wherein heater is a resistor.
18. The device of claim 17, wherein the resistor is a layer of
material on the electro-absorption medium, electrical conductors
are on the base and are in ohmic contact with the layer of
material, and a ratio of a specific electrical resistance of the
layer:a specific electrical resistance of the electrical conductors
is greater than 10:1.
19. The device of claim 1, further comprising: a light source
configured such that the light signal includes at least a portion
of the light generated by the light source; the light source
outputting the light at a channel wavelength and the modulator
configured to modulate the light signal at a modulation wavelength,
the modulation wavelength and the channel wavelength being the same
when the modulator and the light source are both at a design
temperature; and electronics configured to operate the heater such
that a temperature of the heater is maintained above a threshold
temperature, the threshold temperature being less than or equal to
the design temperature.
20. The device of claim 1, wherein a ridge of a light-transmitting
medium is positioned on the base and a ridge of the
electro-absorption medium is positioned on the base, the waveguide
being configured to guide the light signal through both the ridge
of the light-transmitting medium and the ridge of the
electro-absorption medium.
Description
FIELD
[0001] The present invention relates to optical devices and
particularly, to optical devices that include multiple optical
components.
BACKGROUND
[0002] Many communications applications require the linking of
multiple different optical components such as light source and
modulators. For instance, there is a demand for modulators that
modulate a light signal from a light source such as a laser. These
components are often designed so they work well together at a
particular temperature. However, different optical components such
as lasers and modulators generally respond to temperature changes
differently. As a result, two components may operate well together
at one temperature but fail to operate together at other
temperatures. As a result, there is a need for an optical device
that integrates multiple optical components and can be used in a
variety of temperature conditions.
SUMMARY
[0003] An optical device includes a waveguide positioned on a base
and a modulator positioned on the base. The modulator includes an
electro-absorption medium. The waveguide is configured to guide a
light signal through the modulator such that the light signal is
guided through the electro-absorption medium. A heater is
positioned on the electro-absorption medium such that the
electro-absorption medium is between the heater and the base.
BRIEF DESCRIPTION OF THE FIGURES
[0004] FIG. 1A and FIG. 1B illustrates an optical device having a
waveguide that guides a light signal between a light source and a
modulator. FIG. 1A is a perspective view of the device.
[0005] FIG. 1B is a cross section of the device taken along the
line labeled B in FIG. 1A.
[0006] FIG. 2A through FIG. 2E illustrate construction of a
modulator that is suitable for use as the modulator of FIG. 1A.
FIG. 2A is a topview of the portion of the optical device shown in
FIG. 1A that includes an optical modulator.
[0007] FIG. 2B is a cross-section of the optical device shown in
FIG. 1A taken along the line labeled B.
[0008] FIG. 2C is a cross-section of the optical device shown in
FIG. 1A taken along the line labeled C.
[0009] FIG. 2D is a cross-section of the optical device shown in
FIG. 1A taken along the line labeled D.
[0010] FIG. 2E is a cross-section of the optical device shown in
FIG. 1A taken along the line labeled E.
[0011] FIG. 3 is a cross section of an embodiment of a modulator
having a reduced sensitivity to the thickness of the slab regions
on opposing sides of a waveguide.
[0012] FIG. 4A through FIG. 4C illustrate a localized heater in
conjunction with a modulator. FIG. 4A is a topview of the portion
of the device that includes the modulator.
[0013] FIG. 4B is a cross section of the modulator shown in FIG. 4A
taken along the line labeled B in FIG. 4A.
[0014] FIG. 4C is a cross section of the modulator shown in FIG. 4A
taken along the longitudinal axis of the waveguide.
[0015] FIG. 5A is a cross section of a portion of a device that
includes a heater on a modulator. The heater is positioned over the
top and lateral sides of the modulator.
[0016] FIG. 5B is a cross section of a portion of a device that
includes a heater on a modulator.
[0017] FIG. 6A and FIG. 6B illustrate the device of FIG. 4A through
FIG. 4C in combination with the modulator of FIG. 2E. FIG. 6A is a
cross section of the device taken through the modulator.
[0018] FIG. 6B is a cross section of the device taken along the
length of the waveguide.
DESCRIPTION
[0019] An optical device has a modulator that includes an
electro-absorption medium. The device also includes a waveguide
configured to guide a light signal through the electro-absorption
medium included in the modulator. The device also includes a
localized heater that is positioned on at least a portion of the
electro-absorption medium that is included in the modulation. For
instance, the modulator can include a ridge of the
electro-absorption medium and the heater can be positioned on top
of the ridge of electro-absorption medium. Electronics can operate
the heater such that the modulator provides efficient modulation
despite the temperature of the source of the light signal being
anywhere in the full operational temperature range of the device.
Placing the heater on the ridge rather than spaced apart from the
ridge provides a more direct heat transfer to the modulator and
accordingly reduces the energy requirements of the heater. For
instance, simulation results have shown that maximum power usage of
only 54-108 mW per heater can be achieved. It may be possible to
achieve this same result by controlling the temperature of the
entire device through the use of temperature control systems such
as thermo-electric coolers (TEC). However, these temperature
control systems add cost and complexity to the device at the point
of fabrication. Further, these temperature control system have
undesirably large power requirements and are accordingly associated
with ongoing operation costs. As a result, the localized heater can
reduce the costs and power requirements associated with the
device.
[0020] FIG. 1A and FIG. 1B illustrate an optical device having a
waveguide that guides a light signal between a light source 8 and a
modulator 9. FIG. 1A is a perspective view of the device. FIG. 1B
is a cross section of the device taken along the line labeled B in
FIG. 1A. FIG. 1A and FIG. 1B do not show details of either the
light source 8 or the modulator but illustrates the relationship
between these components and the waveguide.
[0021] The device is within the class of optical devices known as
planar optical devices. These devices typically include one or more
waveguides immobilized relative to a substrate or a base. The
direction of propagation of light signals along the waveguides is
generally parallel to a plane of the device. Examples of the plane
of the device include the top side of the base, the bottom side of
the base, the top side of the substrate, and/or the bottom side of
the substrate.
[0022] The illustrated device includes lateral sides 10 (or edges)
extending from a top side 12 to a bottom side 14. The propagation
direction of light signals along the length of the waveguides on a
planar optical device generally extends through the lateral sides
10 of the device. The top side 12 and the bottom side 14 of the
device are non-lateral sides.
[0023] The device includes one or more waveguides 16 that carry
light signals to and/or from optical components 17. Examples of
optical components 17 that can be included on the device include,
but are not limited to, one or more components selected from a
group consisting of facets through which light signals can enter
and/or exit a waveguide, entry/exit ports through which light
signals can enter and/or exit a waveguide from above or below the
device, multiplexers for combining multiple light signals onto a
single waveguide, demultiplexers for separating multiple light
signals such that different light signals are received on different
waveguides, optical couplers, optical switches, lasers that act as
a source of a light signal, amplifiers for amplifying the intensity
of a light signal, attenuators for attenuating the intensity of a
light signal, modulators for modulating a signal onto a light
signal, modulators that convert a light signal to an electrical
signal, and vias that provide an optical pathway for a light signal
traveling through the device from the bottom side 14 of the device
to the top side 12 of the device. Additionally, the device can
optionally, include electrical components. For instance, the device
can include electrical connections for applying a potential or
current to a waveguide and/or for controlling other components on
the optical device.
[0024] A portion of the waveguide includes a first structure where
a portion of the waveguide 16 is defined in a light-transmitting
medium 18 positioned on a base 20. For instance, a portion of the
waveguide 16 is partially defined by a ridge 22 extending upward
from a slab region of the light-transmitting medium as shown in
FIG. 1B. In some instances, the top of the slab region is defined
by the bottom of trenches 24 extending partially into the
light-transmitting medium 18 or through the light-transmitting
medium 18. Suitable light-transmitting media include, but are not
limited to, silicon, polymers, silica, SiN, GaAs, InP and
LiNbO.sub.3.
[0025] The portion of the base 20 adjacent to the
light-transmitting medium 18 is configured to reflect light signals
from the waveguide 16 back into the waveguide 16 in order to
constrain light signals in the waveguide 16. For instance, the
portion of the base 20 adjacent to the light-transmitting medium 18
can be a light insulator 28 with a lower index of refraction than
the light-transmitting medium 18. The drop in the index of
refraction can cause reflection of a light signal from the
light-transmitting medium 18 back into the light-transmitting
medium 18. The base 20 can include the light insulator 28
positioned on a substrate 29. As will become evident below, the
substrate 29 can be configured to transmit light signals. For
instance, the substrate 29 can be constructed of a
light-transmitting medium 18 that is different from the
light-transmitting medium 18 or the same as the light-transmitting
medium 18. In one example, the device is constructed on a
silicon-on-insulator wafer. A silicon-on-insulator wafer includes a
silicon layer that serves as the light-transmitting medium 18. The
silicon-on-insulator wafer also includes a layer of silica
positioned on a silicon substrate. The layer of silica can serving
as the light insulator 28 and the silicon substrate can serve as
the substrate 29.
[0026] Although the light source 8 is shown positioned centrally on
the device, the light source 8 can be positioned at the edge of the
device. The light source 8 can be any type of light source
including light sources that convert electrical energy into light.
Examples of suitable light sources include, but are not limited to,
a semiconductor laser, and a semiconductor amplifier such as a
reflection semiconducting optical amplifier (RSOA). Examples of
suitable lasers include, but are not limited to, Fabry-Perot
lasers, Distributed Bragg Reflector lasers (DBR lasers),
Distributed FeedBack lasers (DFB lasers), external cavity lasers
(ECLs). A variety of suitable lasers and laser constructions are
disclosed in light source applications including U.S. patent
application Ser. No. 13/385,774, filed on Mar. 5, 2012, and
entitled "Integration of Components on Optical Device;" U.S. patent
application Ser. No. 14/048,685, filed on Oct. 8, 2013, and
entitled "Use of Common Active Materials in Optical Components;"
U.S. Provisional Patent Application Ser. No. 61/825,501, filed on
May 20, 2013, and entitled "Reducing Power Requirements for Optical
Links;" U.S. patent application Ser. No. 13/694,047, filed on Oct.
22, 2012, and entitled "Wafer Level Testing of Optical Components;"
U.S. patent application Ser. No. 13/506,629, filed on May 2, 2012,
and entitled "Integration of Laser into Optical Platform;" U.S.
patent application Ser. No. 13/573,892, filed on Oct. 12, 2012, and
entitled "Reduction of Mode Hopping in a Laser Cavity;" U.S. patent
application Ser. No. 13/317,340, filed on Oct. 14, 2011, and
entitled "Gain Medium Providing Laser and Amplifier Functionality
to Optical Device;" U.S. patent application Ser. No. 13/385,275,
filed on Feb. 9, 2012, and entitled "Laser Combining Light Signals
from Multiple Laser Cavities;" each of which is incorporated herein
in its entirety. The light source 8 can be constructed as disclosed
in any one or more of the light source applications and/or can be
interfaced with the device as disclosed in any one or more of the
light source applications. Other suitable light sources include
interdevice waveguides that carry a light signal to the device from
another device such as an optical fiber. A variety of interfaces
between an optical fiber and a device constructed according to FIG.
1A and FIG. 1B are disclosed in fiber interface patents
applications including U.S. patent application Ser. No. 12/228,007,
filed on Nov. 14, 2008, and entitled "Optical System Having Optical
Fiber Mounted to Optical Device," now abandoned; and U.S. patent
application Ser. No. 12/148,784, filed on Apr. 21, 2008, entitled
"Transfer of Light Signals Between Optical Fiber and System Using
Optical Devices with Optical Vias," and issued as U.S. Pat. No.
8,090,231; each of which is incorporated herein in its entirety.
The light source 8 can an optical fiber interfaced with a device as
disclosed in any one or more of the fiber interface patents
applications. In some instances, the device does not include a
light source. For instance, the waveguide can terminate at a facet
located at or near the perimeter of the device and a light signal
traveling through air can then be injected into the waveguide
through the facet. Accordingly, the light source is optional.
[0027] FIG. 2A through FIG. 2E illustrate construction of a
modulator that is suitable for use as the modulator of FIG. 1A.
FIG. 2A is a topview of the portion of the optical device shown in
FIG. 1A that includes an optical modulator. FIG. 2B is a
cross-section of the optical device shown in FIG. 1A taken along
the line labeled B. FIG. 2C is a cross-section of the optical
device shown in FIG. 1A taken along the line labeled C. FIG. 2D is
a cross-section of the optical device shown in FIG. 1A taken along
the line labeled D. FIG. 2E is a cross-section of the optical
device shown in FIG. 1A taken along the line labeled E.
[0028] Recesses 25 (FIG. 2A) extend into the slab regions such that
the ridge 22 is positioned between recesses 25. The recesses 25 can
extend part way into the light-transmitting medium 18. As is
evident from FIG. 2B, the recesses 25 can be spaced apart from the
ridge 22. As a result, a portion of the waveguide 16 includes a
second structure where an upper portion of the waveguide 16 is
partially defined by the ridge 22 extending upward from the slab
region and a lower portion of the waveguide is partially defined by
recesses 25 extending into the slab regions and spaced apart from
the ridge.
[0029] As shown in FIG. 2C, the recesses 25 can approach the ridge
22 such that the sides of the ridge 22 and the sides of the
recesses 25 combine into a single surface 26. As a result, a
portion of a waveguide includes a third structure where the
waveguide is partially defined by the surface 26.
[0030] As is evident in FIG. 2A, a portion of the waveguide 16
includes an electro-absorption medium 27. The electro-absorption
medium 27 is configured to receive the light signals from a portion
of the waveguide having the third structure and to guide the
received light signals to another portion of the waveguide having
the third structure.
[0031] In FIG. 2D, a ridge 22 of electro-absorption medium 27
extends upward from a slab region of the electro-absorption medium
27. Accordingly, a portion of a waveguide includes a fourth
structure configured to guide the received light signal through the
electro-absorption medium 27. This portion of the waveguide is
partially defined by the top and lateral sides of the
electro-absorption medium 27. The slab regions of the
electro-absorption medium 27 and the ridge 22 of the
electro-absorption medium 27 are both positioned on a seed portion
34 of the light-transmitting medium 18. As a result, the seed
portion 34 of the light-transmitting medium 18 is between the
electro-absorption medium 27 and the base 20. In some instances,
when the light signal travels from the light-transmitting medium
into the electro-absorption medium 27, a portion of the light
signal enters the seed portion 34 of the light-transmitting medium
18 and another portion of the light signal enters the
electro-absorption medium 27. As described above, the
electro-absorption medium 27 can be grown on the seed portion of
the light-transmitting medium 18. The seed layer is optional. For
instance, the electro-absorption medium 27 can be grown or
otherwise formed directly on the seed portion of the
light-transmitting medium 18
[0032] As is evident in FIG. 2A, there is an interface between each
facet of the electro-absorption medium 27 and a facet of the
light-transmitting medium 18. The interface can have an angle that
is non-perpendicular relative to the direction of propagation of
light signals through the waveguide 16 at the interface. In some
instances, the interface is substantially perpendicular relative to
the base 20 while being non-perpendicular relative to the direction
of propagation. The non-perpendicularity of the interface reduces
the effects of back reflection. Suitable angles for the interface
relative to the direction of propagation include but are not
limited to, angles between 80.degree. and 89.degree., and angles
between 80.degree. and 85.degree..
[0033] The optical device includes a modulator. The location of the
modulator on the optical device is illustrated by the line labeled
K in FIG. 2A. In order to simplify FIG. 2A, the details of the
modulator construction are not shown in FIG. 2A. However, the
modulator construction is evident from other illustrations such as
FIG. 2E. The modulator of FIG. 2E is constructed on the portion of
the waveguide having a fourth structure constructed according to
FIG. 2D. The perimeter of portions of doped regions shown in FIG.
2E are illustrated with dashed lines to prevent them from being
confused with interfaces between different materials. The
interfaces between different materials are illustrated with solid
lines. The modulator is configured to apply an electric field to
the electro-absorption medium 27 in order to phase and/or intensity
modulate the light signals received by the modulator.
[0034] A ridge 22 of the electro-absorption medium 27 extends
upward from a slab region of the electro-absorption medium 27.
Doped regions 40 are both in the slab regions of the
electro-absorption medium 27 and also in the ridge of the
electro-absorption medium 27. For instance, doped regions 40 of the
electro-absorption medium 27 are positioned on the lateral sides of
the ridge 22 of the electro-absorption medium 27. In some
instances, each of the doped regions 40 extends up to the top side
of the electro-absorption medium 27 as shown in FIG. 2E.
Additionally, the doped regions 40 extend away from the ridge 22
into the slab region of the electro-absorption medium 27. The
transition of a doped region 40 from the ridge 22 of the
electro-absorption medium 27 into the slab region of the
electro-absorption medium 27 can be continuous and unbroken as
shown in FIG. 2E.
[0035] Each of the doped regions 40 can be an N-type doped region
or a P-type doped region. For instance, each of the N-type doped
regions can include an N-type dopant and each of the P-type doped
regions can include a P-type dopant. In some instances, the
electro-absorption medium 27 includes a doped region 40 that is an
N-type doped region and a doped region 40 that is a P-type doped
region. The separation between the doped regions 40 in the
electro-absorption medium 27 results in the formation of PIN
(p-type region-insulator-n-type region) junction in the
modulator.
[0036] In the electro-absorption medium 27, suitable dopants for
N-type regions include, but are not limited to, phosphorus and/or
arsenic. Suitable dopants for P-type regions include, but are not
limited to, boron. The doped regions 40 are doped so as to be
electrically conducting. A suitable concentration for the P-type
dopant in a P-type doped region includes, but is not limited to,
concentrations greater than 1.times.10.sup.15 cm.sup.-3,
1.times.10.sup.17 cm.sup.-3, or 1.times.10.sup.19 cm.sup.-3, and/or
less than 1.times.10.sup.17 cm.sup.-3, 1.times.10.sup.19 cm.sup.-3,
or 1.times.10.sup.21 cm.sup.-3. A suitable concentration for the
N-type dopant in an N-type doped region includes, but is not
limited to, concentrations greater than 1.times.10.sup.15 cm',
1.times.10.sup.17 cm', or 1.times.10.sup.19 cm', and/or less than
1.times.10.sup.17 cm', 1.times.10.sup.19 cm', or 1.times.10.sup.21
cm.sup.3.
[0037] Electrical conductors 44 are positioned on the slab region
of the electro-absorption medium 27. In particular, the electrical
conductors 44 each contact a portion of a doped region 40 that is
in the slab region of the electro-absorption medium 27.
Accordingly, the each of the doped regions 40 is doped at a
concentration that allows it to provide electrical communication
between an electrical conductor 44 and one of the doped regions 40
in the electro-absorption medium 27. As a result, electrical energy
can be applied to the electrical conductors 44 in order to apply
the electric field to the electro-absorption medium 27. The region
of the light-transmitting medium or electro-absorption medium
between the doped regions can be undoped or lightly doped as long
as the doping is insufficient for the doped material to act as an
electrical conductor that electrically shorts the modulator.
[0038] During operation of the modulators of FIG. 1A through FIG.
2E, electronics 47 (FIG. 1A) can be employed to apply electrical
energy to the electrical conductors 44 so as to form an electrical
field in the electro-absorption medium 27. For instance, the
electronics can form a voltage differential between the doped
regions that act as a source of the electrical field in the gain
medium. The electrical field can be formed without generating a
significant electrical current through the electro-absorption
medium 27. The electro-absorption medium 27 can be a medium in
which the Franz-Keldysh effect occurs in response to the
application of the electrical field. The Franz-Keldysh effect is a
change in optical absorption and optical phase by an
electro-absorption medium 27. For instance, the Franz-Keldysh
effect allows an electron in a valence band to be excited into a
conduction band by absorbing a photon even though the energy of the
photon is below the band gap. To utilize the Franz-Keldysh effect
the active region can have a slightly larger bandgap energy than
the photon energy of the light to be modulated. The application of
the field lowers the absorption edge via the Franz-Keldysh effect
and makes absorption possible. The hole and electron carrier
wavefunctions overlap once the field is applied and thus generation
of an electron-hole pair is made possible. As a result, the
electro-absorption medium 27 can absorb light signals received by
the electro-absorption medium 27 and increasing the electrical
field increases the amount of light absorbed by the
electro-absorption medium 27. Accordingly, the electronics can tune
the electrical field so as to tune the amount of light absorbed by
the electro-absorption medium 27. As a result, the electronics can
intensity modulate the electrical field in order to modulate the
light signal. Additionally, the electrical field needed to take
advantage of the Franz-Keldysh effect generally does not involve
generation of free carriers by the electric field.
[0039] Suitable electro-absorption media 27 for use in the
modulator include semiconductors. However, the light absorption
characteristics of different semiconductors are different. A
suitable semiconductor for use with modulators employed in
communications applications includes Ge.sub.1-xSi.sub.x
(germanium-silicon) where x is greater than or equal to zero. In
some instances, x is less than 0.05, or 0.01. Changing the variable
x can shift the range of wavelengths at which modulation is most
efficient. For instance, when x is zero, the modulator is suitable
for a range of 1610-1640 nm. Increasing the value of x can shift
the range of wavelengths to lower values. For instance, an x of
about 0.005 to 0.01 is suitable for modulating in the c-band
(1530-1565 nm).
[0040] A modulator having a cross section according to FIG. 2E can
be sensitive to the thickness of the slab regions of the
electro-absorption medium 27. For instance, as the thickness of the
slab region increases, the ridge becomes smaller and the electrical
field formed between the doped regions 40 accordingly fills a
smaller portion of the distance between the base 20 and the top of
the ridge. For instance, the location of the electrical field
effectively moves upwards from the base 20. The increased space
between the electrical field and the base 20 can be thought of as
increasing the resistance or carrier diffusion time of the
modulator. This increase in resistance and/or diffusion time
decreases the speed of the modulator. Problems also occur when
these slab regions become undesirably thin. When these slab regions
become thin, the doped regions extend down into the
light-transmitting medium 18. This doping of the light-transmitting
medium 18 also decreases the speed of the modulator.
[0041] FIG. 3 presents an embodiment of a modulator having a
reduced sensitivity to the thickness of the slab regions. The
perimeter of portions of doped regions shown in FIG. 3 are
illustrated with dashed lines to prevent them from being confused
with interfaces between different materials. The interfaces between
different materials are illustrated with solid lines. A first doped
zone 46 and a second doped zone 48 combine to form each of the
doped regions 40. In some instance, the first doped zone 46 is
located in the light-transmitting medium 18 but not in the
electro-absorption medium 27 and the second doped zone 48 is
located in the electro-absorption medium 27. The first doped zone
46 can contact the second doped zone 48 or can overlap with the
second doped zone 48. In some instances, the first doped zone 46
and the second doped zone 48 overlap and at least a portion of the
overlap is located in the light-transmitting medium 18. In other
instances, the first doped zone 46 and the second doped zone 48
overlap without any overlap being present in the electro-absorption
medium 27.
[0042] The first doped zone 46 and the second doped zone 48
included in the same doped region 40 each includes the same type of
dopant. For instance, the first doped zone 46 and the second doped
zone 48 in an n-type doped region 40 each includes an n-type
dopant. The first doped zone 46 and the second doped zone 48
included in the same doped region 40 can have the same dopant
concentration or different concentrations.
[0043] Although FIG. 3 illustrates the slab regions including the
electro-absorption medium 27, the slab regions of the
electro-absorption medium 27 may not be present. For instance, the
etch that forms the slab regions of the electro-absorption medium
27 may etch all the way through the slab regions. In these
instances, the first doped zone 46 and the second doped zone 48 are
both formed in the light-transmitting medium.
[0044] Although FIG. 3 shows the first doped zone 46 not extending
down to the light insulator 28, the first doped zone 46 can extend
down to the light insulator 28 or into the light insulator 28.
[0045] The above modulators can include a localized heater
configured to heat all or a portion of the modulator. The localized
heaters are not illustrated in FIG. 2A through FIG. 3 in order to
illustrate the parts that underlay the heater. However, FIG. 4A
through FIG. 4C illustrate the localized heater in conjunction with
a modulator. The details of the modulator are not illustrated, but
the modulator can be constructed according to FIG. 2E or FIG. 3 or
can have another construction. FIG. 4A is a topview of the portion
of the device that includes the modulator. FIG. 4B is a cross
section of the modulator shown in FIG. 4A taken along the line
labeled B in FIG. 4A. FIG. 4C is a cross section of the modulator
shown in FIG. 4A taken along the longitudinal axis of the waveguide
16.
[0046] The heater 50 is on the ridge 22 such that the modulator is
positioned between the heater 50 and the base. One or more layers
of material can optionally be positioned between the heater and the
ridge. For instance, the heater 50 can be located on an insulating
layer 52 that electrically insulates the heater from the underlying
layers. The insulating layer 52 is positioned between the heater
and the ridge 22. Suitable insulating layers 52 include, but are
not limited to, silica and silicon nitride. An insulating layer
with a higher thermal conductivity may be preferred in or to
provide a pathway from heat to travel from the heater to the
modulator. Accordingly, insulating layers 52 that are thinner
and/or have a higher thermal conductivity may be desired. In some
instances, the insulating layer 52 has a thermal conductivity above
10 W/mK.
[0047] One or more claddings 54 are optionally positioned between
the waveguide 16 and the insulating layer 52 and/or between the
waveguide 16 and the heater 50. At least one of the claddings 54
can directly contact the light-transmitting medium 18. A cladding
that contacts light-transmitting medium 18 preferably has a lower
index of refraction than the light-transmitting medium 18. When the
light-transmitting medium 18 is silicon, suitable claddings
include, but are not limited to, polymers, silica, SiN and
LiNbO.sub.3. In some instances, a single layer of material can
serve as both a cladding 54 and an insulating layer 52. Although
the insulating layer 52 is shown as a single layer of material, the
insulating layer 52 can include or consist of multiple layers of
material.
[0048] Conductors 56 are positioned so as to provide electrical
communication between the heater 50 and contact pads 58. The
conductors 56 and contact pads 58 can be electrically conducting.
The electronics 47 can apply electrical energy to the contact pads
58 so as to deliver electrical energy to the heater 50 and can
accordingly operate the heater so the heater 50 generates heat. The
location of the heater on the ridge 22 allows the generated heat to
elevate the temperature of the ridge through a mechanism such as
conduction.
[0049] In some instances, the heater 50 is an "electrical
resistance heater." For instance, the heater 50 can include or
consist of an electrically conducting layer 60 that serves as a
resistor. An example of a suitable resistor is a trace that
includes or consists of a metal, metal alloy. Examples heaters
include or consist of titanium traces, tungsten titanium traces,
and nichrome traces. During operation of the device, the
electronics 47 can drive sufficient electrical current through the
electrically conducting layer 60 to cause the electrically
conducting layer 60 to generate the heat that is conducted to the
modulator. The conductors 56 can include or consist of an
electrically conductive layer 62 and can be arranged such that the
electrical current flows parallel or substantially parallel to the
ridge 22 or the direction of light signal propagation through the
ridge. As a result, the length of the ridge 22 that is heated by
the heater can be increased merely by increasing the length of the
resistor.
[0050] The electrically conducting layer 60 can have a higher
resistance/length than the electrically conductive layers 62 in
order to stop or reduce generation of heat by the conductors 56.
This can be achieved by using different materials and/or dimensions
for the electrically conductive layer 62 and the conducting layer
60. For instance, the electrically conductive layer 62 can be
aluminum while the conducting layer 60 that serves as the heater is
titanium. Titanium has a specific electrical resistance of about 55
.mu.ohm-cm while aluminum has a specific electrical resistance of
about 2.7 .mu.ohm-cm. As a result, the conductors 56 and conducting
layer 60 can have similar cross sectional dimensions and an
electrical current can be driven through the conductors 56 and
conducting layer such that heat is generated at the conducting
layer without undesirable levels of heat being generated by the
conductors 56. Alternately, the conductors 56 can have larger cross
section dimensions than the heater in order to further reduce heat
generation by the conductors 56
[0051] In some instances, the conductors 56 include a conducting
layer 60 from the heater 50 in addition the conductive layer 62 as
is evident in FIG. 4B. In these instances, the conductive layer 62
can be more conductive and/or have larger dimensions than the
conducting layer 60 in order to reduce generation of heat by the
conductor 56. When the conductors 56 includes the conducting layer
60 and the conductive layer 62, the conductors 56 and heater can be
formed by forming a first layer of the material for the conducting
layer and then forming a second layer of material for the
conductive layer over the first layer. Suitable methods for forming
the first layer and the second layer on the device include, but are
not limited to, sputtering, XXX, XXX and XXX. The first layer and
the second layer can then be patterned so as to form the conductors
and heater on the device. Suitable methods for patterning include,
but are not limited to, etching in the presence of one or more
masks. The portion of the second layer over the heater 50 can then
be removed to provide the configuration of conducting layer and
conductive layer shown in FIG. 4A and FIG. 4B. Suitable methods for
removing the portion of the second layer include, but are not
limited to, etching in the presence of a mask. Although the
electrically conducting layer 60 and the electrically conductive
layers 62 are disclosed as a single layer of material, either or
both of the electrically conducting layer 60 and the electrically
conductive layers 62 can include or consist of one or more
different layers of material.
[0052] A suitable ratio for the specific electrical resistance of
the conducting layer 60:conductive layer 62 is greater than 5:1,
10:1, or 50:1.
[0053] FIG. 4A through FIG. 4C illustrate the heater 50 as being
positioned on the top of the electro-absorption medium or on top of
the ridge 22. Additionally or alternately, the heater can be
positioned on one or more lateral sides of the electro-absorption
medium or on one or more lateral sides of the ridge 22. For
instance, FIG. 5A is a cross section of the device such as the
cross section of FIG. 4B. FIG. 5A illustrates the heater positioned
on both the top and lateral sides of the ridge 22. As a result, the
heater is positioned on both the top and lateral sides of the
electro-absorption medium 27. In some instances, the heater 50 is
positioned on one or more of the lateral sides of the
electro-absorption medium 27 without being positioned on the top of
the electro-absorption medium 27 and/or on one or more of the
lateral sides of the ridge 22 without being positioned on the top
of the ridge 22. The heater does not extend down to the base of the
ridge but can extend all the way to the base of the ridge.
[0054] The heater 50 can extend away from the ridge 22 such that
the heater 50 is positioned over the slab regions. For instance,
FIG. 5B is a cross section of the modulator where the heater is
positioned on the ridge of the electro-absorption medium 27,
extends down to the base of the ridge 22, and extends away from the
base of the ridge 22 on the slab regions. The distance that the
heater extends away from the ridge is labeled E in FIG. 5B. The
distance is equal to the distance between the edge of the heater
and the portion of the heater on the lateral side of the ridge 22.
Increasing the distance that the heater extends away from the ridge
can reduce the degree of localized heating and can increase the
power requirements for the device. In some instances, the distance
that the heater extends away from the ridge is less than 2 .mu.m, 1
.mu.m, or 0.5 .mu.m and can be 0 .mu.m. The bottom or lower side of
the heater 50 is between the top (or upper side) of the heater 50
and the modulator 9 and/or the electro-absorption medium 27. In
some instances, the heater 50 is arranged such that the bottom (or
lower side) of the heater 50 does not contact the device at a
location that is more than 2 .mu.m, 200 .mu.m, or 500 .mu.m away
from a lateral side of the ridge and/or an edge of the heater is
not located more than 2 .mu.m, 200 .mu.m, or 500 .mu.m away from
the nearest lateral side of the ridge. In other words, no portion
of the heater through which heat travels to the device is located
more than 2 .mu.m, 200 .mu.m, or 500 .mu.m away from the nearest
lateral side of the ridge or the heater is not positioned over a
location that is more than 2 .mu.m, 200 .mu.m, or 500 .mu.m away
from the nearest lateral side of the ridge.
[0055] In FIG. 4A through FIG. 5B, the bottom (lower side) of the
heater 50 is between the top of the heater 50 and the modulator 9
and/or the electro-absorption medium 27. Moving the bottom of the
heater 50 closer to the electro-absorption medium 27 and/or the
ridge 22 reduces the distance over which the generated heat must be
conducted in order to elevate the temperature of the modulator and
can accordingly reduce the amount of heat that must be generated in
order to achieve a particular temperature within the modulator.
Reducing the thickness of the one or more layers of material
between the bottom of the heater and the electro-absorption medium
27 can move the bottom of the heater 50 closer to the
electro-absorption medium 27. For instance, reducing the thickness
of the one or more claddings 54 and the one or more insulating
layers 52 can move the bottom of the heater 50 closer to the
electro-absorption medium 27. In some instances, all or a portion
of the bottom of the heater 50 is within 0.5, 1, or 2 .mu.m of the
electro-absorption medium 27.
[0056] The details of the modulator construction are not
illustrated in FIG. 4A through FIG. 5B; however, the modulator can
have a variety of constructions including, but not limited to, the
constructions of FIG. 2E or FIG. 3. In order to illustrate this
concept, FIG. 6A and FIG. 6B illustrate the device of FIG. 4A
through FIG. 4C in combination with the modulator of FIG. 2E. FIG.
6A is a cross section of the device taken through the modulator.
FIG. 6B is a cross section of the device taken along the length of
the waveguide. The heater 50 is positioned over at least a portion
of the electro-absorption medium 27 that is included in the
modulator such that the electro-absorption medium 27 is located
between the heater 50 and the base. FIG. 6B shows that the heater
50 does not extend beyond the perimeter of the electro-absorption
medium 27; however, one or both ends of the electro-absorption
medium 27 can extend beyond the perimeter of the electro-absorption
medium 27.
[0057] As is evident in FIG. 6A, a protective layer 64 can
optionally be formed over the above devices. In some instances, the
protective layer 64 can have a thermal conductivity that is less
than the thermal conductivity of the one or more claddings 54
and/or the one or more insulating layers 52. The reduced thermal
conductivity of the protective layer 64 causes heat generated by
the heater to be directed toward the modulator and can accordingly
reduce the energy requirements of the heater as well as reduce
thermal cross talk. Suitable protective layers include, but are no
limited to, silica, silicon nitride, and aluminum oxide. Although
the protective layer is disclosed as a single layer of material,
the protective layer can be constructed of multiple layers of
material. In some instances, one, two or three layers of the
protective layer have a thermal conductivity greater than 0.75
WK/m, 1.0 WK/m, or 1.25 WK/m. The protective layer is not
illustrated in FIG. 6B.
[0058] The modulator of FIG. 4A through FIG. 5B can have
constructions other than the constructions of FIG. 1A through FIG.
3. Examples of other suitable modulator constructions can be found
in U.S. patent application Ser. No. 12/653,547, filed on Dec. 15,
2009, entitled "Optical Device Having Modulator Employing
Horizontal Electrical Field," and U.S. patent application Ser. No.
13/385,774, filed on Mar. 4, 2012, entitled "Integration of
Components on Optical Device," each of which is incorporated herein
in its entirety. U.S. patent application Ser. Nos. 12/653,547 and
13/385,774 also provide additional details about the fabrication,
structure and operation of these modulators. In some instances, the
modulator is constructed and operated as shown in U.S. patent
application Ser. No. 11/146,898; filed on Jun. 7, 2005; entitled
"High Speed Optical Phase Modulator," and now U.S. Pat. No.
7,394,948; or as disclosed in U.S. patent application Ser. No.
11/147,403; filed on Jun. 7, 2005; entitled "High Speed Optical
Intensity Modulator," and now U.S. Pat. No. 7,394,949; or as
disclosed in U.S. patent application Ser. No. 12/154,435; filed on
May 21, 2008; entitled "High Speed Optical Phase Modulator," and
now U.S. Pat. No. 7,652,630; or as disclosed in U.S. patent
application Ser. No. 12/319,718; filed on Jan. 8, 2009; and
entitled "High Speed Optical Modulator;" or as disclosed in U.S.
patent application Ser. No. 12/928,076; filed on Dec. 1, 2010; and
entitled "Ring Resonator with Wavelength Selectivity;" or as
disclosed in U.S. patent application Ser. No. 12/228,671, filed on
Aug. 13, 2008, and entitled "Electrooptic Silicon Modulator with
Enhanced Bandwidth;" or as disclosed in U.S. patent application
Ser. No. 12/660,149, filed on Feb. 19, 2010, and entitled "Reducing
Optical Loss in Optical Modulator Using Depletion Region;" each of
which is incorporated herein in its entirety. A review of the
modulators disclosed in these applications shows that the slab
regions of the electro-absorption medium 27 are optional. The
heater 50, one or more insulating layers 52, one or more claddings
54, and conductors 56 can be fabricated using fabrication
technologies that are employed in the fabrication of integrated
circuits, optoelectronic circuits, and/or optical devices.
[0059] The device can also include one or more temperature sensors
(not shown) that are each positioned to sense the temperature of
the modulator and/or the temperature of a zone adjacent to the
modulator. Suitable temperature sensors include, but are not
limited to, thermocouples, thermistors, integrated PN diodes, or
other integrated semiconductor devices.
[0060] The electronics can adjust the level of electrical energy
applied to the heater in response to the output received from the
one or more temperature sensors in a feedback loop. For instance,
the electronics can operate the heater such that the temperature of
the heater stays at or above a threshold temperature (T.sub.th)
during operation of the device. For instance, when the electronics
determine that the temperature of the modulator falls below the
threshold temperature, the electronics can apply electrical energy
to the heater so as to bring the temperature of the modulator to or
above the threshold temperature. However, when the electronics
determine that the temperature of the modulator falls above the
threshold temperature, the electronics can refrain from applying
the electrical energy to the heater. As a result, when the
electronics determine that the temperature of the modulator is
above the threshold temperature, the temperature of the modulator
can float in response to the operation of the device in the ambient
atmosphere.
[0061] The device is configured to operate over an operational
ambient temperature range. For instance, the device should be able
to continue operating when the ambient temperature in which the
device is positioned (TA) extends from TL to TH. In some instances,
TL is below 0.degree. C., 10.degree. C., or 20.degree. C. and/or TH
is greater than 50.degree. C., 70.degree. C., or 80.degree. C. The
operational ambient temperature range is typically from
TL=0.degree. C. to TH=70.degree. C. The operational temperature
range is generally defined as part of the specification for the
device. In general the operational temperature range is designed so
the device meets customer requirements.
[0062] The width of the band of wavelengths that can be efficiently
modulated by a modulator is the operating bandwidth (OBW) of the
modulator. The operating bandwidth is generally the length of the
band of wavelengths where the modulator has low insertion loss and
high extinction ratio at a particular temperature. For a Franz
Keldysh modulator constructed according to FIG. 2E, the operating
bandwidth (OBW) is generally about 35 nm. The operating bandwidth
(OBW) for a modulator can be identified by applying a modulation
signal to the modulator and measuring the response of the optical
signal through the modulator over a range of wavelengths. The range
of wavelengths for which the insertion loss and high extinction
ratio produce loss of less than 1 dB can serve as the operating
bandwidth. In some instances, the range of wavelengths for which
the insertion loss and high extinction ratio produce loss of less
than 1.5 dB or 2.0 can serve as the operating bandwidth. In some
instances, the operating wavelength range for a modulator is more
than 25 nm, 30 nm, or 35 nm and/or less than 40 nm, 50 nm, or 60
nm.
[0063] The wavelength at the center of the operating bandwidth
(OBW) is considered the modulation wavelength. The wavelengths that
fall within the operating bandwidth (OBW) shifts in response to
temperature changes; however, the operating bandwidth (OBW) stays
constant or substantially constant. As a result, the modulation
wavelength is a function of temperature but the operating bandwidth
(OBW) can be approximate as being independent of temperature. The
rate that the modulation wavelength of the above modulators shifts
in response to temperature changes (.DELTA..lamda..sub.m) is about
0.76 nm/.degree. C. and the operating bandwidth (OBW) stays
substantially constant at about 35 nm.
[0064] The most intense wavelength produced by the light source is
considered the channel wavelength of the light signal produced by
the light source. The light source and the modulator are generally
configured to operate together at a design temperature (TT). For
instance, the light source and modulator are generally configured
such that the modulation wavelength and the channel wavelength are
the same at the design temperature. As a result, the modulator
efficiently modulates the output of the light source at the design
temperature. The design temperature is generally equal to a common
temperature for the ambient environment in which the device is
positioned. A typical design temperature is 60.degree. C. In some
instances, the design temperature serves as the threshold
temperature (T.sub.th).
[0065] The channel wavelength and the modulation wavelength at the
design temperature are the design wavelength (.lamda..sub.T). The
modulation wavelength at a particular temperature can be expressed
relative to the design wavelength. For instance, the modulation
wavelength at a particular temperature can be expressed as
.lamda..sub.T-(TT-T.sub.m)(.DELTA..lamda..sub.m) where T.sub.m
represents the temperature of the modulator.
[0066] The channel wavelength shifts in response to changes in the
temperature of the light source (T.sub.LS). For instance, the
channel wavelength shift rate for a light source
(.DELTA..lamda..sub.LS) such as a DFB laser is generally about 0.08
nm/.degree. C. at 1550 nm and for a Fabry-Perot laser is generally
about 0.5 nm/.degree. C. The wavelength of the light source at a
particular temperature can be expressed as follows:
.lamda..sub.T-(TT-T.sub.LS)(.DELTA..lamda..sub.LS). Other suitable
light sources have a rate of modulation wavelength shift greater
than 0.05, 0.1, or 0.2 nm/.degree. C. and/or less than 0.3, 0.5, or
0.7 nm .degree. C.
[0067] Variables in the fabrication process generally produce
modulators having a range of modulation wavelengths at a particular
temperature. For instance, a batch of modulators will generally
have modulation wavelengths that are equal to the desired
modulation wavelengths+/-a manufacturing tolerance. The
manufacturing tolerance can be indicated by a multiple of the
standard deviation. For instance, a Franz Keldysh modulator
constructed according to FIG. 2E generally has a manufacturing
tolerance (MT) of about 7.5 nm where 7.5 nm represents three times
the standard deviation. The presence of this manufacturing
tolerance reduces the amount that the wavelength of a light signal
being received by the modulator can shift while still reliably
falling within the operating bandwidth (OBW) for each of the
modulators. For instance, a light signal that shifts by less than a
permissible range (PR) will still reliably have a wavelength that
falls within the operating bandwidth (OBW) of a modulator
fabricated with the above manufacturing tolerance and can
accordingly be efficiently modulated by the modulator. The
permissible range (PR) can be determined as ((OBW-2MT)/2).
[0068] The difference between the modulation wavelength and the
channel wavelength must be less than or equal to the permissible
range (PR) of the modulator in order for the modulator to reliably
provide efficient modulation of the light signal. Accordingly,
under these conditions, it can be stated that
[.lamda..sub.T-(TT-T.sub.m)(.DELTA..lamda..sub.m)]-[.lamda..sub.T-(TT-T.s-
ub.LS)(.DELTA..lamda..sub.LS)].ltoreq.PR or
(TT-T.sub.LS)(.DELTA..lamda..sub.LS)-(TT-T.sub.m)(.DELTA..lamda..sub.m).l-
toreq.PR. Solving for T.sub.LS provides that
T.sub.LS.gtoreq.TT-[PR-(TT-T.sub.m)(.DELTA..lamda..sub.m)]/(.DELTA..lamda-
..sub.LS). When the electronics hold the temperature of the
modulator constant at T.sub.th, this expression becomes
T.sub.LS.gtoreq.TT-[PR-(TT-T.sub.th)(.DELTA..lamda..sub.m)]/(.DELTA..lamd-
a..sub.LS). In instances where the threshold temperature is equal
to the design temperature (TT), this expression reduces to
T.sub.LS.gtoreq.TT-[PR/(.DELTA..lamda..sub.LS)] or
T.sub.LS.gtoreq.TT-[(OBW/2-MT)/(.DELTA..lamda..sub.LS)]. Using the
above numbers for a DFB laser where the threshold temperature is
equal to a design temperature of 60.degree. C. shows that the light
source temperature (T.sub.LS) can fall as low as [60.degree.
C.-[(35 nm/2-7.5 nm)]/(0.08 nm/.degree. C.)]=-65.degree. C. before
the channel wavelength falls outside of the permissible range (PR)
of the modulator. Accordingly, efficient modulation of the light
signal produced by the light source can still be achieved when the
light source temperature (T.sub.LS) drops to -65.degree. C.
However, TL is generally about 0.degree. C. As a result, the
threshold temperature can actually be reduced below the design
temperature. For instance, a threshold temperature of 54.degree. C.
permits the light source temperature (T.sub.LS) to fall as low as
-8.degree. C. before the channel wavelength falls outside of the
permissible range (PR) of the modulator. The ability of the
threshold temperature to be below the design temperature reduces
the power requirements associated with the heater.
[0069] As noted above, the electronics can refrain from operating
the heater when the temperature of the modulator would be above the
threshold temperature without the operation of the heater.
Substituting the above numbers into
(TT-T.sub.LS)(.DELTA..lamda..sub.LS)-(TT-T.sub.m)(.DELTA..lamda..sub.m).l-
toreq.PR shows that the temperature of the light source and the
modulator can concurrently be as high as about 74.degree. C. while
still having a wavelengths that fall within the permissible range
(PR). However, the upper end of the operational ambient temperature
range (TH) is generally about 70.degree. C. As a result, the
operation of the modulator and light sensor can drive the
temperature of both of these components up by an additional
4.degree. C. while still achieving efficient modulation of the
light signal. Accordingly, the method of operating the heater
provides efficient light signal modulation across the entire
operational ambient temperature range (TH).
[0070] Simulation results have shown that for a heater that is 20
.mu.m long used with a modulator having a ridge with of 1 .mu.m, a
ridge height of 2.7 .mu.m, and a slab region thickness of 0.3
.mu.m, the power requirements for a heater constructed as disclosed
above are about 1-2 mW/.degree. C. Accordingly, when the
temperature of a modulator would be at 0.degree. C. without
operation of the heater, a power in a range of 60 to 120 mW would
be needed to keep the temperature of the modulator at a threshold
temperature of 60.degree. C. and a power of only about 54 to 108 mW
would be needed to keep the temperature of the modulator at a
threshold temperature of 54.degree. C. Since 0.degree. C. is
generally the bottom of the operational ambient temperature range,
the maximum power requirement for the heater is less than 120 mW,
108 mW, 80 mW, 60 mW or 54 mW.
[0071] Although the device is disclosed as having a single
modulator and heater, this is for illustrative purposes and a
single device will often have more than one modulator that includes
a heater constructed and/or operated as disclosed above. Examples
of a single device that includes multiple light sources and
multiple modulators can be found in U.S. patent application Ser.
No. 14/048,685, filed on Oct. 8, 2013, and entitled "Use of Common
Active Materials in Optical Components" and in other patent
applications that are incorporated into this disclosure. Different
heaters on a single device can be operated using the same method
variables or using different method variables. For instance,
different heaters can be operated with different threshold
temperatures or can be operated with the same threshold
temperature. Accordingly, the different modulators can be at
different temperatures.
[0072] Although FIG. 1A and FIG. 1B illustrate a waveguide that
connects the light source directly with a modulator, the device
need not include a light source as is disclosed above. Further, the
device can be constructed such that the modulator receives a light
signal that includes at least a portion of the light generated from
one or more light sources. Accordingly, other components can be
optically between the light source and the modulator. For instance,
the device can include a multiplexer that multiplexes light signals
from multiple light sources into a second light signal that is
received by the modulator constructed as disclosed above.
Additionally or alternately, the device can include a demultiplexer
that receives a light signal from multiple different light sources
and demultiplexes the light signal into multiple second light
sources such that at least one of the second light signals is
received by the modulator constructed as disclosed above.
Accordingly, multiplexers and demultiplexers can be positioned
between a light source and a modulator that receives at least a
portion of the light output from the light sensor. Other examples
of components that can be optically between a light source and a
modulator that receives at least a portion of the light output by
the light source include, but are not limited to, amplifiers,
switches, combiners, splitters, y-junctions, optical taps, in-line
photodetectors and polarization rotators.
[0073] Although the above heater is disclosed as generating heat
through the application of electrical energy to the heater, other
heating mechanisms can be employed. For instance, the heater can
guide a heated liquid or can be a source of a light.
[0074] Although the device is disclosed in the context of a
silicon-on-insulator platform, the device can be constructed on
other platforms.
[0075] Although the above modulators are disclosed as having a
single heater, a modulator can include more than one heater or more
than one heating element. For instance, a heater can include
multiple resistors connected in series or in parallel.
[0076] Although the heater is disclosed as being positioned on the
ridge of a modulator, the heater can be positioned on the ridge of
other optical components such as light sensors and light sources
such as are disclosed in U.S. patent application Ser. No.
13/506,629.
[0077] Other embodiments, combinations and modifications of this
invention will occur readily to those of ordinary skill in the art
in view of these teachings. Therefore, this invention is to be
limited only by the following claims, which include all such
embodiments and modifications when viewed in conjunction with the
above specification and accompanying drawings.
* * * * *